When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Anti-arrhythmic drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems often include portions that are implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is determining whether the therapy delivered has had its desired effect. For example, after implanting a pacer in a patient, a physician or other caregiver would like to know if the pace pulses being delivered are effective at “capturing the heart,” i.e., evoking a contraction of the heart chamber to which the pace pulse was delivered. If the paces are not succeeding at capturing the heart, the physician will likely program a higher energy pace pulse to obtain capture. In order to save energy, prolonging the useful life of the implanted device before replacement is required, lower energy paces are preferable provided that the physician is assured that the lower energy pace pulses will capture the heart. Replacement of the implanted device carries significant expense as well as some risk of discomfort and/or complications.
In order to determine the appropriate energy of pacing therapy, the physician typically programs several different therapy energy levels (i.e., pacing voltage amplitude, pacing pulsewidth, or combination of amplitude and pulsewidth) to see what energy levels appropriately obtain capture. Because proper therapy energy levels are critical in providing effective cardiac rhythm management therapy and extending the useful life of the implanted device, there is a need for techniques that assist the physician or other caregiver in determining threshold energies for cardiac rhythm management therapy.